Where is the viewport render resolution divider?
this patch description (linked from the above D1619 thread) says:

"Cycles/BI: Add a pixel size option for speeding up viewport rendering
This patch adds "Pixel Size" to the performance options, which allows to render in a smaller resolution, which is especially useful for displays with high DPI."

about the UI part the patch shows a new property, called "preview_pixel_size":

this patch  was released Aug 15 2017, but 2.79 final was released september 12 so it couldn't be set for that release, but it is in following releases, such as current "unstable" build blender-2.79-c66983c-win64.zip, in which you can find it where the patch said, under "performance options":